1. Field of the Invention
The present invention relates to a coupler for use in association with single mode fibers and, more particularly, to such a coupler which comprises an appropriate length of multimode fiber connected to the end of the single mode fiber wherein the section of multimode fiber performs as a lens.
2. Description of the Prior Art
A common problem in the design of single mode optical components is the need to provide efficient and stable optical coupling. Typically, this coupling is between two single mode optical fibers located at different ports on a given device. A standard method of coupling two single mode fibers requires the expansion of the input beam passing through an input single mode fiber using a lens, often a graded-index (GRIN) lens. The expanded beam then passes through the optical component(s) after which a second lens focuses the beam onto the core of the output single mode fiber. One such arrangements disclosed in U.S. Pat. No. 4,193,663 issued to C. Timmermann on Mar. 18, 1980. In this arrangement, a lens is adhered to the end of the fiber, where the lens is made of a material having a melting point which is low in relation to the melting point of the fiber, resulting in a light guide having an essentially semispherical lens at the end. In one embodiment, the lens is made of epoxy in order to facilitate the adhesion to the fiber. The use of a GRIN lens as a coupling mechanism between two fibers is disclosed in U.S. Pat. No. 4,268,112 issued to K. P. Peterson on May 19, 1981. Several different embodiments are disclosed which are based on the optical transmission characteristics of a Luneberg lens. In particular, a spherical "bead" of graded-index material is used, where the bead is formed around a wire, for example, tungsten, in order to leave a precision hole through the center of the bead. Input and output fibers are then inserted at opposite ends of this hole, where the ends of the fibers are positioned at the focal points of the spherical bead. Both of these arrangements require that the axes of the lenses intersect the center of the fiber core and that the lenses themselves be situated in the plane of the fiber junction. An alternative prior art arrangement which substantially mitigates these difficulties is disclosed in U.S. Pat. No. 4,327,963 issued to G. D. Khoe et al on May 4, 1982. In this arrangement a convex spherical graded index lens is used which is rotationally symmetric about any axis through its center. Therefore, alignment between the fiber and the lens is somewhat simplified, yet the center of the lens must still be aligned with the core of the fiber.
A problem remaining with these and other prior art coupling designs is the sensitivity to misalignment at the fiber/lens interface. This misalignment creates both the need for precise micropositioning during fabrication and the need to maintain a stable fiber/lens bond while the device is in use.